1. Field of the Invention
The present invention relates to motor drive technology.
2. Description of the Related Art
1. In recent years, with the speeding up of personal computers and workstations, operating speeds of LSIs (Large Scale Integrated circuits) for processing in CPUs (Central Processor Units), DSPs (Digital Signal Processors), and the like, have been increasing.
In such LSIs, as the operating speeds, that is, clock frequencies, become higher, amounts of heat generated become large. There is a problem in that the generated heat from an LSI leads to thermorunaway in the LSI itself, or impacts on neighboring circuits. Therefore, appropriate heat cooling of the LSIs has become a very important technology.
As an example of technology for cooling the LSI, there is a cooling method comprising an air-cooling system by a cooling fan. In this method, for example, the cooling fan is installed facing a surface of the LSI, and cool air is blown on the LSI surface by the cooling fan. When the LSI is cooled by this type of cooling fan, temperature in the vicinity of the LSI is monitored, and the cooling level is adjusted by changing rotations of the fan in accordance with the temperature (Patent Documents 1 and 2). Furthermore, Patent Document 3 discloses technology in which a duty ratio of a pulse-width modulated signal is limited, so that a fan motor rotates at or above a predetermined minimum number of rotations.
2. Technology is widely used for controlling a conduction time of a coil of a motor, by a pulse signal such as a pulse-width modulation signal or the like, in order to rotate a single phase motor or a polyphase motor at a desired torque.
When starting driving of a stopped motor, when a switching voltage, which has a pulse width corresponding to a target torque, is suddenly applied to the motor coil, a current flows rapidly in the motor coil. At startup of the motor, particularly when the number of rotations thereof is close to 0, since there is no power generation capability, the current flowing in the coil has a value of applied voltage divided by winding resistance. The resistance value of the coil is designed to be very low, in order to reduce power losses, and there are cases in which the coil current exceeds a rating of a drive circuit or the coil itself, and has an effect on reliability of the circuit.
Furthermore, back electromotive force (back electromotive voltage) that occurs in the coil is proportional to time variation rate of the current flowing in the coil. Therefore, as described above, when the coil current rapidly increases, a very large back electromotive voltage occurs, and there is a risk of the rating of the drive circuit being exceeded.
For this reason, immediately after initiating startup of the motor, soft start control is performed to gradually increase the current flowing in the coil (for example, refer to Patent Documents 1 and 2). In the soft start control described in Patent Document 1, a soft start voltage, in which the voltage value is gradually increased with time, is generated; this soft start voltage is compared to a cyclic voltage of a triangular waveform or a sawtooth waveform; a pulse-modulation signal whose duty ratio gradually increases is generated; and conduction time of the coil is gradually increased, to perform the soft start.
Patent Document 1: Japanese Patent Application, Laid Open No. H07-31190
Patent Document 2: Japanese Patent Application, Laid Open No. 2001-284868
Patent Document 3: Japanese Patent Application, Laid Open No. 2004-153955
Patent Document 4: Japanese Patent Application, Laid Open No. H06-98574
Patent Document 5: Japanese Patent Application, Laid Open No. H07-95792
Patent Document 6: Japanese Patent Application, Laid Open No. 2001-45790
1. There has been a problem in that, when a rotating motor is stopped, a regenerative current flows in a coil, and this regenerative current flows towards a power supply. Cases are envisioned in which back electromotive voltage generated at this time exceeds withstanding voltage of a transistor device or the like. This problem occurs not only with driving a fan motor, but also with driving other motors.
Patent Document 4 discloses technology in which a thyristor is arranged in parallel with a motor that is to be driven, and regenerative control is performed. However, in the technology described in Patent Document 4, a bridge circuit is configured of a plurality of transistors, and the technology cannot be used when the plural transistors are driven, being alternately turned ON and OFF, (for example, a H-bridge circuit or the like of Patent Document 3).
In a motor drive circuit using the H-bridge circuit or the like, in order to protect the circuit from back electromotive voltage that occurs when the motor is stopped, a method is adopted in which a capacitor is arranged between a power supply voltage terminal and ground, or a zener diode is provided, and an escape path for the regenerative current is formed, and the back electromotive voltage is clamped. However, there has been a problem in that addition of the capacitor or the zener diode leads to an increase in circuit area and cost.
2. Furthermore, in soft start control described in Patent Documents 4 and 5, there is also a problem in that conduction time of the motor can only be increased monotonically.